1. Field of the Invention
This invention relates generally to chemical mechanical planarization, and more particularly to a non-coherent profiled retaining ring for reducing non-uniformity during a chemical mechanical planarization process.
2. Description of the Related Art
In the fabrication of semiconductor devices, planarization operations are often performed, which can include polishing, buffing, and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide.
As semiconductor fabrication is an automated process, techniques have been developed to ensure fabrication robots properly align wafers within each step of wafer fabrication. For example, wafers are often notched at a point along the edge of the wafer to facilitate proper wafer alignment. Other alignment techniques include the use of flatted wafers, wherein an edge of the wafer is flat (not rounded). However, as described in greater detail subsequently, flatted wafers often generate problems during particular wafer manufacturing processes, such as during wafer planarization.
As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal planarization operations are performed to remove excess metallization. Further applications include planarization of dielectric films deposited prior to the metallization process, such as dielectrics used for shallow trench isolation or for poly-metal insulation. One method for achieving semiconductor wafer planarization is the chemical mechanical planarization (CMP) process.
In general, the CMP process involves holding and rubbing a typically rotating wafer against a moving polishing pad under a controlled pressure and relative speed. CMP systems typically implement orbital, belt, or brush stations in which pads or brushes are used to scrub, buff, and polish one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
As mentioned above, techniques have been developed to ensure fabrication robots properly align wafers within each step of wafer fabrication. Conventional CMP systems often have little trouble when polishing notched wafers. Unfortunately, conventional CMP systems generally do not perform satisfactorily when processing flatted wafers.
FIG. 1 is a diagram showing a conventional carrier head 100 holding a flatted wafer 102. As illustrated in FIG. 1, the wafer 102 is held in position during CMP processing by a conventional retaining ring 104, which surrounds the wafer 102. Generally, a small distance delta exists between the edge of the wafer 102 and the interior surface of the retaining ring 104 to allow the wafer 102 to be easily positioned within the carrier head 100. During a CMP operation the carrier head 100 rotates in a direction 110 along a polishing belt or table, depending on the type of CMP system utilizing the carrier head 100. As mentioned above, the polishing surface moves beneath the wafer 102 during polishing.
The movement of the polishing surface causes a friction force 106, which is applied to the wafer 102. Because of the delta between the wafer 102 and the retaining ring 104, the friction force 106 pushes the wafer 102 in the direction of the polishing surface movement until the wafer is stopped by the retaining ring 104. Once the wafer 102 contacts the retaining ring 104, a reaction force 108 is generated from the retaining ring 104. Generally, the reaction force 108 does not contributed greatly to uniformity errors when the rounded edges of the wafer 102 come into contact with the retaining ring 104. However, because of the delta between the wafer 102 and the retaining ring 104, the wafer 102 rotates within the retaining ring 104. As a result, the comers of the flatted portion of the wafer 102 eventually come into contact with the retaining ring 104, as illustrated in FIG. 2.
FIG. 2 is an illustration showing prior art carrier head 100 when the flatted section of the wafer 102 contacts the conventional retaining ring 104. As above, the wafer 102 is held in position by the conventional retaining ring 104, which surrounds the wafer 102. However, as shown in FIG. 2, the wafer 102 has rotated such that two corners 200 of the flatted section of the wafer 102 are both in contact with the retaining ring 104.
The contact of the two corners 200 with the retaining ring 104 generates reaction forces 202 concentrated at the comers 200 of the flatted section of the wafer 102. As is well known to those skilled in the art, each reaction force 202 can be split into component forces 204a and 204b for easier analysis. In particular, each reaction force 202 comprises a first force component 204a, which is directed along the rounded edge of the wafer 102, and a second force component 204b, which is directed along the flatted edge of the wafer 102. Hence, the second force components 204b of the reaction force 202 from each comer 200 are opposed to each other, causing stress to wafer 102 from the corners 200. Unfortunately, the opposing second force components 204b cause the wafer 102 buckle near the flatted section, as shown by area 206. As a result, the buckled flatted wafer section 206 is pushed into the polishing surface, causing over-polishing in the flatted wafer section 206 as illustrated in FIG. 3.
FIG. 3 is an illustration showing a flatted wafer 102 resulting from a CMP operation using a conventional retaining ring. When the flatted wafer section 206 is buckled and, as a result, pushed into the polishing surface, non-uniformity results. In particular, the flatted area 206 of the wafer 102 is polished with an increased removal rate relative to the remaining sections of the wafer 102 because of the additional force present in the flatted area 206 during polishing. As a result, the flatted area 206 of the wafer 102 is over-polished. The resulting non-uniformity can have a dramatic negative effect on the devices formed on the wafer, often causing the entire wafer to be discarded.
In view of the foregoing, there is a need for CMP techniques and apparatuses that allow flatted wafers to be polished with an essentially uniform removal rate. In particular, the apparatuses should not allow over-polishing of the flatted section and should allow essentially uniform planarization during a CMP process.
Broadly speaking, the present invention fills these needs by providing a non-coherent profiled retaining ring that allows planarization of flatted wafers without over-polishing the flatted region of the wafer. In one embodiment, a retaining ring for use in a CMP system is disclosed that includes an annular retaining ring capable of holding a flatted wafer in position during a CMP operation. The flatted wafer has a first corner and a second corner disposed on a flatted edge of the wafer. The retaining ring further comprises a plurality of profiled teeth disposed along an interior surface of the annular retaining ring. The profiled teeth are separated from each other such that the first corner and the second corner of the wafer do not contact profiled teeth simultaneously at all orientations of the wafer in the retaining ring. In addition, the profiled teeth can be further separated such that a predefined variation in length of the flatted edge of the wafer will not cause the first comer and the second comer to contact profiled teeth simultaneously at all orientations of the wafer in the retaining ring. In this manner, embodiments of the present invention can account for wafer size variation.
An additional retaining ring for use in a CMP is disclosed in an additional embodiment of the present invention. As above, the retaining ring includes an annular retaining ring capable of holding a flatted wafer in position during a CMP operation. Also as above, the flatted wafer has a first comer and a second comer disposed on a flatted edge of the wafer. In addition, a plurality of profiled teeth is included that are disposed along an interior surface of the annular retaining ring. In this embodiment, a surface of each tooth that contacts the wafer is inclined so as to form an angle greater than 90xc2x0 relative to a polishing surface and away from the center of the wafer. That is, an edge of the surface of each tooth that contacts the wafer closest to the polishing surface can also be closest to a center of the wafer. In this manner, the surface of each tooth that contacts the wafer can be inclined such that a lifting force is generated during the CMP operation that pushes the wafer in a direction away from the polishing surface.
A further retaining ring is disclosed for use in a CMP system in a further embodiment of the present invention. The retaining ring includes an annular retaining ring capable of holding a flatted wafer in position during a CMP operation. As above, the flatted wafer has a first corner and a second corner disposed on a flatted edge of the wafer. Also included is a plurality of profiled teeth disposed along an interior surface of the annular retaining ring. The profiled teeth are separated from each other such that the first corner and the second comer of the wafer do not contact profiled teeth simultaneously at all orientations of the wafer in the retaining ring. In addition, a surface of each tooth that contacts the wafer is inclined so as to form an angle greater than 90xc2x0 relative to a polishing surface and away from the center of the wafer. Similar to above, the profiled teeth can be further separated such that a predefined variation in length of the flatted edge of the wafer will not cause the first comer and the second comer to contact profiled teeth simultaneously. Also as above, the surface of each tooth that contacts the wafer can be inclined such that a lifting force is generated during the CMP operation that pushes the wafer in a direction away from the polishing surface. Advantageously, each embodiment of the present invention can be utilized in a linear wafer planarization apparatus, and/or a table base wafer planarization apparatus.
Embodiments of the present invention advantageously avoid wafer bending, and thus over-polishing, by eliminating the two corner reaction force interaction during CMP operations. Furthermore, since the profiled teeth disposed completely around the interior surface of the retaining ring, this is true at all orientations of the wafer in the retaining ring. Moreover, by inclining the internal surfaces of the retaining ring, such as the ring itself or the profiled teeth of the profiled retaining ring, embodiments of the present invention can reduce friction force and edge effect. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.